GAAP is based on some basic underlying principles and concepts such as the cost principle, matching principle, full disclosure, going concern, economic entity, conservatism, relevance, and reliability. (You can learn more about the basic principles in Explanation of Accounting Principles.)
GAAP, however, is not static. It includes some very complex standards that were issued in response to some very complicated business transactions. GAAP also addresses accounting practices that may be unique to particular industries, such as utility, banking, and insurance. Often these practices are a response to changes in government regulations of the industry.